In electronic communications, electromagnetic signals carry information between two nodes over a connecting medium. The signal strength at the receiving node varies depending on the distance between the nodes and changes in the condition of the medium. For instance, the signal strength typically decreases with increasing distance between the two nodes. Furthermore, even if the distance is fixed, physical variations in the medium over time can affect signal strength. For example, in a cable system, different cables can have different attenuation constants. Also, changes in atmospheric conditions between transmitting and receiving antennas in a broadcast system, or multipath interference due to reflections from moving objects such as cars or airplanes, or changes in the gains of repeater amplifiers in cable television systems due to outdoor temperature changes can affect the signal strength at the receiver. Finally, variations in transmitter output power will also affect signal strength at the receiver.
An automatic gain control (AGC) circuit and a variable gain amplifier (VGA) are often used at the receiver input to compensate for variations of received signal strength. More specifically, the AGC circuit adjusts the gain setting of the VGA to maintain the signal strength within a desired operating range. If the received signal strength is too high, then the AGC lowers the gain setting of the VGA. If the received signal strength is too low, then the AGC raises the gain setting of the VGA.
In order to properly compensate a received signal, the VGA should have a dynamic range that is equivalent to, or greater than, the range of variation in the received signal. In addition, the VGA should have a gain step size that is fine enough to avoid adding to much quantization noise to the received signal that would prevent useful information from being extracted. For example, quadrature amplitude modulation (QAM) with a 256-point constellation (256 QAM) is often used in cable television systems and requires a VGA step size to be less than about 0.03 dB for operation.
When a large gain control range (e.g., 40 dB) is required simultaneously with fine gain steps (e.g., 0.03 dB), continuously controllable VGAs are often employed. A continuously controllable VGA requires a continuous control voltage that is noise-free and has a very fine voltage resolution. Often, sigma-delta digital-to-analog converters (DACs) are used to provide this continuous control voltage. However, a drawback of sigma-delta DACs is that they generally require filtering components that cannot be integrated on chip with the remaining components of the VGA.
Therefore, what is needed is a PGA, with very fine gain steps and a large gain range.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.